The invention relates to triggering the optimization of a link between network elements controlling the radio channels of a mobile communication system and a mobile switching centre as a result of handover, and particularly to when the optimization is triggered.
Cellular communication systems are characterized in that mobile stations can freely move and connect from one cell to another within the area of the mobile communication system. Crossover is just a re-registration to a new cell, when the mobile station does not have an ongoing call. If a mobile station has an ongoing call during crossover, the call must also be connected from one base station to another with as little disturbance to the call as possible. Crossover performed during a call is called handover. Handover may also take place within a cell from one traffic channel to another.
Several different types of handovers can be distinguished depending on which elements of the mobile communication system participate in handover. In the following, handover types are described using the IMT-2000 system (International Mobile Telecommunication 2000) as an example system. It is one of the third-generation mobile communication systems. Another example of a corresponding system is Universal Mobile Communication System (UMTS). UMTS is being standardized in ETSI (European Telecommunication Standards Institute), whereas the IMT-2000 system is being standardized by ITU (International Telecommunication Union). These future systems are very similar in basic features. The network architecture of the IMT-2000 system is illustrated in FIG. 1. It describes the parts of the system that are most important for handover. Mobile switching centres MSC connect incoming and outgoing calls and control the radio network controllers RNC of their areas. In the following, the mobile switching centre and the radio network controller are connected by referring to the mobile switching centre as the radio network controller""s own mobile switching centre. For instance, in FIG. 1, the radio network controllers RNC1 and RNC2 have the same own mobile switching centre MSC1 and the radio network controller RNC3""s own mobile switching centre is MSC2. The radio network controllers RNC are responsible for the control functions related to radio access and control the base transceiver stations BTS in their area, through which the mobile stations are connected to the network over radio paths. The details of the network structure bear no essential significance for the invention.
There are three basic types of handover. When a mobile station remains in the area of the same radio network controller RNC, the handover is called handover between base transceiver stations. When the radio network controller area changes, but the mobile station remains within the area of the same mobile switching centre MSC, the handover is called internal handover within a mobile switching centre. When the mobile station moves from the area of the first mobile switching centre MSC-1 to the area of a second mobile switching centre MSC-2, the handover is called handover between mobile switching centres. Additionally, systems using code division multiple access (CDMA), such as the known third-generation mobile communication systems currently being developed, handover is either soft or hard. In a soft handover, the mobile station can during handover be simultaneously connected to the network through several base transceiver stations, until one of them proves to be better than the others on the basis of its signal. In a hard handover, the mobile station is connected to the network through only one base transceiver station.
When a call is made to or from a mobile station MS, a connection is established between the mobile station and a mobile switching centre MSC. The connection is established through a radio network controller RNC. This radio network controller is called the serving RNC and other radio network controllers are drift RNCs. The serving RNC is also called an anchor RNC. The serving RNC acts for instance as a combining point, i.e. it handles the diversity combining in soft handover. The connection to the switching centre is established by creating an lu link for the call, i.e. a speech channel between the serving RNC and its own mobile switching centre. During handover, the serving RNC can be maintained by means of an interface lur between the radio network controllers. A drift RNC can also be a serving RNC at the same time acting for instance as a combining point in soft handovers. According to present knowledge, when handover is completed between two radio network controllers RNC, the drift RNC becomes the serving RNC from which a new lu link is created for the call, and the old link is released. An alternative solution is that the radio network controller which is the first serving radio network controller stays as the serving radio network controller and a new lu link is not created.
The problem with the above solution conforming to present knowledge is the continuous change of lu link, if the mobile station moves back and forth between two radio network controllers and causes an lu link optimization after each handover, or alternatively moves quickly from one area to another. This is illustrated in the example in FIG. 1, in which a mobile station MS makes a call while being in area A1 and moves to area A2 during the call as indicated with the arrow. The serving RNC is first RNC1 and the lu link is lu1. The mobile station moves to the area of radio network controller RNC2 and the lu link is optimized to lu2. Next, the mobile station moves to the area of radio network controller RNC3 and the lu link is optimized to lu3. If the move happens quickly, the optimization of the lu link to lu2 uses up network resources unnecessarily. Correspondingly, if the mobile station moves back and forth, for instance between the areas of base transceiver stations BTS1 and BTS2, and causes an optimization of the lu link with every handover, network resources would be unnecessarily used. Again, if the mobile station stays long under the control of radio network controller RNC2, network resources can be saved by optimizing lu2 as the lu link.
The problem with the alternative solution is that network resources are unnecessarily used if the lu link is not optimized again. Network resources are especially misspent in the example in FIG. 1 when the mobile station moves from area A1 to area A2, if lu1 remains as the lu link even if it is under the control of a different mobile switching centre. The problem is emphasized, if the RNC-RNC connections are not built separately and they utilize the same transmission resources as the MSC-RNC connections.
It is thus an object of the invention to develop a method and an apparatus implementing the method, which solve the above-mentioned problems and perform the lu link optimization just at the right time in a manner which saves network resources. This object is achieved by a method, a system, a controller and a mobile station characterized by what is disclosed in the independent claims. The preferred embodiments of the invention are set forth in the dependent claims.
The invention is based on the idea that information, i.e. history data, is collected concerning handovers of a mobile station and after certain conditions based on the history data are fulfilled, the lu link between the mobile switching centre and a radio network controller is optimized again.
The method and system of the invention provide the advantage that unnecessary optimization of the lu link is avoided and the link is optimized only when the optimization saves network resources. The mobile station performs handovers between radio network controllers normally, but to the network side the handover between the radio network controllers, i.e. the lu link optimization, is performed in a manner which optimizes the use of network resources.
In a preferred embodiment of the invention, the history data includes network information that is updated in connection with each handover. This provides the advantage that the conditions triggering optimization can be made versatile to better take different handovers into consideration.
In a further preferred embodiment of the invention, the history data comprises a handover counter which is updated in connection with handovers between controllers. By means of the counter, it is very simple to detect handovers between controllers and react to them.